Noise is an unwanted by-product of electronic circuit operation. It is especially troublesome in digital circuits and power switching circuits. While it is impossible to completely obliterate noise, it is possible through careful circuit design to reduce the quantum of noise to a level which does not interfere with satisfactory circuit operation in a particular application. Of course, reducing noise to lower and lower levels incurs substantially increasing costs. Accordingly, in designing circuits, and especially digital and power switching circuits, it is important to quantify the noise present in any circuit and, in certain applications, determine any significant noise source in the circuit so that the most effective noise reduction steps can be implemented. In other applications, it is desirable to inject noise into a circuit under test to determine the noise margin, i.e., the amount of additional noise which must be added before circuit operation ceases to meet prescribed performance objectives. This margin is important, as it sets guidelines on the environment in which a circuit may operate.
Prior art techniques to measure noise rely on direct connection or physical contact between the probe leads of an oscilloscope or multimeter and the circuit under test. Injection of noise signals was also accomplished through direct connection of a signal generator and the circuit under test. The problem with this technique is that the resulting measurements will vary with the location at which the ground connection of the probe is affixed. In addition, direct connection of the probe as a means of injecting noise is not always possible without disturbing circuit operation to such an extent as to render any measurements meaningless. In light of the foregoing, improvements in the apparatus for noise measurements would be extremely desirable.